The present invention pertains to a method for manufacturing photomasks, which are particularly used in the manufacture of microelectronic components with submicronic dimensions. Furthermore, it extends to the device for implementing this method.
Semiconductor substrates, particularly those made of silicon, are currently micro-machined using plasma etching techniques based on a pattern transferred onto the substrate from a mask. A photomask is equivalent to a photographic negative: it contains information to be printed onto a medium. It is generally used as a transmission method for exposures and printing onto semiconductor substrates. Various parameters, including the focusing wavelength, define the depth of the active area, which is printed directly onto the substrate. Outside of the active area, the details are not printed, but may have an impact on the transmission of the photomask. Pollution in the active area has a direct effect on the image printed onto the substrate, because a defect will be printed. However, this pollution only has an indirect effect on this image if it occurs outside of this area, such as lowering the contrast or reducing the transmission of the photomask.
Furthermore, the semiconductor industry is researching how to reduce the dimensions of the printed image in order to obtain electronic components which are increasingly small and integrable, and less expensive. The dimensions of photomasks are becoming smaller, while pollution requirements are becoming ever stricter. The photomask is therefore a key, expensive, and complex element which should be kept clean and operational.
The active surface of the photomasks must be free of all particles, particularly in the focal plane, as these contaminating particles create a defect which is printed and which is reproduced on the semiconductor substrate. At the end of its manufacturing process, the mask is cleaned, and then a pellicle is applied to the photomask in order to protect its active surface from any particles. The purpose of the pellicle is to protect the photomask during its lifetime of use by the user. The pellicle consists of an optical membrane deposit (parallel multilayer surfaces) with a good transmission and a reduced impact on the optical rays which pass through it. This pellicle is deposited beside the active surface of the photomask, and separated from it by a space. The pollutants that could accumulate onto the active surface of the photomask will thereby accumulate onto the pellicle, outside of the focus area (physically separated from the active surface). In this way, these pollutants will not be printed during the lithographic transfer: the pellicle does not directly protect against particulate pollutants, but it makes possible to reduce their impact on the image.
The document US-2001/005944 pertains to eliminating gas contaminants that came from the surrounding atmosphere, such as O2, CO2 and H2O, which may be present in the space between the photomask and the pellicle. These contaminants are particularly harmful, as they prevent the photolithographic operation from proceeding properly by inhibiting the transmission of radiation at 157 nm, which is normally used. Decontamination takes place in a sealed chamber, in a vacuum or in an optically inert gas, via exposure to UV radiation, plasma, ozone, and/or heat. This treatment causes the gas desorption to accelerate.
However, the increase in energy needed for exposure in order to create ever smaller patterns has resulted in a new problem. The gases found beneath the pellicle, such as ammonia, fluorine, and volatile organic compounds, combine when exposed to this high energy to create crystals which grow over time. The crystals which appear beneath the pellicle, and therefore in the focus area, produce defects in the printed area on the substrate. These crystals represent a major problem, because they cause numerous unforeseeable defects on the substrates, and may affect more than 20% of the most advanced current photomasks.
One of the chemical reactions that occur may be summarized by the following formula:H2SO4+NH3+hν→(NH4)2SO4 solid 
Ammonia (NH3) comes from multiple sources, but essentially from human activity in the photomask manufacturing areas, and from the use of these photomasks. With the goal of reducing the phenomenon of crystal growth, semiconductor and photomask manufacturers have made massive investments to limit the ammonia found in the clean room, and have set strategies for storing and transporting photomasks in protected environments in order to reduce their contact with the ammonia.
Sulfuric acid (H2SO4) is commonly used by photomask manufacturers during the steps of the manufacturing process, particularly during etching and stripping operations. In the most common occurrence, the last step of the cleaning process, before the pellicle placement step, requires sulfates and generates sulfate residue. The last step of the stripping process, which consists of removing the previously applied resin and which comes before cleaning, also generates sulfate residue. This sulfate residue trapped beneath the pellicle will therefore desorb, which is the main reason why these crystals appear in silicon chip manufacturers' plants. Photomask manufacturers make an effort to reduce the quantity of sulfates used during the cleaning steps, by modifying the methods or by adding steps intended to reduce sulfate residue content. However, these new methods or the mitigating techniques that are instituted are costlier and less effective, and it is not possible to completely eliminate the usage of sulfates during the steps of the manufacturing process.
The pellicle is put into place after a stripping step followed by a cleaning step. The last cleaning step is carried out in a clean area. This pellicle will make it possible to keep the particles generated in the clean room or in the production equipment from accumulating onto the active surface of the photomask.
One of the mitigating solutions consists of periodically inspecting the photomask's active surface. As soon as the first crystals appear, the photomask is returned to its manufacturer. The pellicle is removed and cleaned, and then a new pellicle is applied to the photomask. This operation must be carried out by the photomask manufacturers and not by the users, which causes wasted time and major additional costs for managing the stock due to the photomasks' shortened lifespan.